The present invention relates to actuators for moving the intake and exhaust valves of internal combustion engines, and specifically to an electronically actuable engine valve providing improved force characteristics and a signal indicating the valve position.
Electrically actuable valves, in contrast to valves actuated mechanically by cams and the like, allow a computer-based engine controller to easily vary the timing of the valve opening and closing during different phases of engine operation.
One type of actuator for such a valve provides a flat plate armature which moves back and forth between two electromagnets. The armature is attached to a valve stem of a valve.
When the electromagnets are unpowered, the armature is held in equipoise between the two electromagnets by two opposing springs. Prior to operation, the armature is drawn against one of the electromagnets by an "initialization" current in the retaining electromagnet. The spring between the armature and the retaining electromagnet is compressed while the opposing spring is stretched. Once the armature is drawn fully toward the receiving electromagnet, the initialization current is reduced to a "holding" level sufficient to hold the armature against the electromagnet until the next transition is initiated.
A change of valve state from open to closed or vice versa, is effected by interrupting the holding current. When this occurs, the energy stored in the opposed compressed and stretched springs accelerates the armature off of the releasing electromagnet toward the new receiving electromagnet. When the armature reaches the receiving electromagnet, that electromagnet is energized with the "holding" current to retain the armature in position against its surface.
In a frictionless system, the armature reaches a maximum velocity at the midpoint between the two electromagnets (assuming equal spring forces) and just reaches the receiving electromagnet with zero velocity. In a physically realizable system in which friction causes some of the stored energy of the springs to be lost as heat, the armature will not reach the receiving electromagnet unless the energy lost to friction is replaced. This is accomplished by creating a "capture" current in the receiving electromagnet prior to the armature contacting that electromagnet.
The capture current must be of sufficient magnitude to overcome the opposing forces resisting movement of the armature, however, it is equally important that the capture current be limited to prevent damage to the armature, electromagnet, or valve and to limit impact noise. If the capture current is turned on too soon (or is too great in magnitude), the armature may be accelerated into the electromagnet (and the valve into its seat) at excessive velocity. Conversely, the armature may not be captured by the receiving electromagnet and the valve may not close if the capture current is turned on too late or is too low in magnitude.
Accurate control of the capture current is facilitated if the position and velocity of the armature as it approaches the receiving electromagnet can be measured. Because the force between the electromagnet and armature varies rapidly with distance, sensors for measuring armature distance must be very accurate. Small measurement errors in distance can produce large errors in the calculated force applied to the armature, upsetting correct armature control.
Unfortunately, position sensors that are sufficiently accurate for this purpose and yet robust enough to survive in the environment of an internal combustion engine are expensive and therefore impractical.